1. Field of the Invention
This invention relates to tensioned leg offshore platforms of the type used in oil and gas drilling and production and relates more particularly to an anchor apparatus located at the lower end of the tension legs of the tension leg platform that anchors the tension leg platform to the ocean floor.
2. Background of the Invention
As offshore exploration for oil and gas from subsea deposits has expanded into deeper and deeper waters, conventional rigid towers setting upon the ocean floor and extending upward to the surface have become more and more impractical.
One particular solution to this problem which has been proposed is the elimination of the rigid tower and the substitution therefor of a floating platform moored to the ocean floor by a plurality of vertical members which are placed under high tension loads due to excess buoyancy of the floating platform. Examples of such structures, which are generally referred to as tension leg platforms, are shown in U.S. Pat. No. 3,648,638 to Blenkarn and U.S. Pat. No. 3,919,957 to Ray, et al.
Typically, such a tension leg platform is designed to have a plurality of spaced clusters of vertical tension legs or tethering elements, each of said clusters including a plurality of tethers arranged in a predetermined pattern with all of the clusters additionally arranged in a predetermined pattern relative to each other, so that all of the tethers are arranged substantially vertically between the surface platform and anchors located upon the ocean floor. It is desirable that such tethers be vertically arranged, with no substantial skewing from a vertical line, because the tension variations created in the tethering elements by the forces exerted thereupon by the buoyant surface structure are greater if the tethers are non-vertical as compared to perfectly vertical.
This requires that the relative locations of all of the anchoring positions for the lower ends of the tethers be accurately located upon the ocean floor. The prior art shows several approaches to achieving this goal.
One approach is to construct a single unitary very large structure to be placed upon the ocean floor, which structure includes the anchoring points for each of the tethers and also generally includes connections for production risers or the like, which are to be connected to the floating platform. This solves the problem of providing accurate relative positioning of the tethers, but creates another problem in that the extremely large unitary anchor structure is difficult and expensive to manufacture, transport to the offshore drilling site, and install at a desired location upon the ocean floor. One such structure is shown in U.S. Pat. No. 3,611,734 to Mott.
Another somewhat different solution to this problem is to construct a unitary anchor structure, portions of which are hinged so as to allow the structure to partially collapse to thereby make it easier to transport, while still maintaining a predetermined spacing of the tether attachment points due to the non-variable relative positioning of those points once the structure is expanded to its final orientation. Such a structure is shown in U.S. Pat. No. 4,126,008 to Dixon.
The use of separate anchors for each cluster of tethering elements has been suggested in U.S. Pat. No. 3,919,957 to Ray, et al.; U.S. Pat. No. 3,982,492 to Steddum, and U.S. Pat. No. 3,996,755 to Kalinowski. In the Steddum and Ray, et al. patents the separate anchors are lowered to the ocean floor directly from the floating structure. The Kalinowski patent merely refers to the anchors as being preplaced without specifying any particular manner for locating and orienting the anchors.
An apparatus and method need to be disclosed that combines all of the more desirable features of the above references. The apparatus should, instead of the single massive unitary anchor structure, be relatively easy to transport and connect to the ocean floor. The apparatus should be simpler in design and fabrication than the hinged structure as disclosed in Dixon. And the time required in connecting separate anchor clusters to individual tethers should also be minimized, due to the possible exposure of the tension leg platform and its crew to oncoming storms while partially anchored to the ocean floor.